Heat Exchanger, In Particular Exhaust Gas Heat Exchanger

ABSTRACT

The invention relates to a heat exchanger, in particular an exhaust gas heat exchanger, having at least one surface which is impinged on by a medium, in particular exhaust gas, is made from metal, in particular aluminum or stainless steel, and is provided with a coating. In order to improve the properties of the coating, according to the invention, the coating comprises a coating material based on nanotechnology.

The invention relates to a heat exchanger, in particular an exhaust gas heat exchanger, having at least one metal surface, in particular an aluminum or special steel surface, on which a medium, in particular exhaust gas, impinges and which is provided with a coating. The invention also relates to a method of manufacturing a heat exchanger as described above.

In exhaust gas heat exchangers, exhaust gas, predominantly from diesel engines, together with moisture and heat, leads to corrosion attacks on the metals used. Heat-resistant paints may be used to protect against corrosion.

The object of the invention is to create a heat exchanger, in particular an exhaust gas heat exchanger, having at least one metal surface, in particular an aluminum or special steel surface, on which a medium, in particular exhaust gas, impinges and which is provided with a coating, said coating possessing better characteristics than conventional paints, and manufacture of the heat exchanger being reliable.

In a heat exchanger, in particular an exhaust gas heat exchanger, having at least one metal surface, in particular an aluminum or special steel surface, on which a medium, in particular exhaust gas, impinges and which is provided with a coating, the object is achieved in that the coating comprises a coating material, which is based on nanotechnology. The coating material preferably comprises at least one nanomaterial or nanostructure. The coating is vitreous and possesses a very good chemical resistance.

A preferred exemplary embodiment of the heat exchanger is characterized in that the coating comprises a main constituent, which is composed of an organic and an inorganic fraction. It is possible, by way of the cross-linking temperature, to define and to vary the characteristics of the coating within wide limits.

A further preferred exemplary embodiment of the heat exchanger is characterized in that the coating contains silicon. Organo(alkoxy)silanes are preferably purposely hydrolyzed by the use of suitable catalysts, eliminating alcohols.

Further preferred exemplary embodiments of the heat exchanger are characterized in that the coating contains titanium, zirconium, aluminum, magnesium, zinc and/or calcium. The inorganic network can be purposely modified by the various substances.

In a method of manufacturing a heat exchanger as described above, in particular an exhaust gas heat exchanger, the aforesaid object is achieved in that the coating is produced by a sol-gel process. In the sol-gel process a sol is converted into a gel to produce nano-materials. Through hydrolysis and condensation reactions a three-dimensional network of interlayered molecules is produced in a liquid. Thermal processing stages serve to process the gels further into nano-materials or nanostructures.

A preferred exemplary embodiment of the method is characterized in that at least one sol is applied to the surface to be coated. The surface to be coated may be wetted with the sol in any suitable way.

A further preferred exemplary embodiment of the method is characterized in that the sol is cured. The curing is preferably performed under the effect of heat.

A further preferred exemplary embodiment of the method is characterized by the following process steps: a heat exchanger to be coated is flooded with the coating material and drained; the drained heat exchanger is heated in a drying oven. The heat exchanger to be coated is force-flooded by the coating substance and then drained. The heat exchanger is then preferably suspended so that all excess coating substance can run off without unwanted accumulations of coating substance being formed inside the heat exchanger. Drops adhering to the outlet from the heat exchanger are suitably removed, for example by means of compressed air or with the aid of an electrostatic droplet extractor.

Further advantages, features and details of the invention are set forth in the following description, in which various exemplary embodiments are described in detail. The features mentioned in the claims and in the description may here each be essential for the invention either individually or in any combination.

The invention relates to an exhaust gas heat exchanger made from aluminum or special steel. The exhaust gas heat exchanger has a cavity, through which exhaust gas flows when the exhaust gas heat exchanger is in operation. The cavity is coated with a nanotechnology-based coating substance. The main constituent of the coating substance is composed of an organic and an inorganic fraction. It is possible, by way of the cross-linking temperature, to define the characteristics of the coating within wide limits. At high stoving temperatures a higher proportion of the organic contents is expelled, that is to say there is a greater degree of crosslinking. The corrosion resistance of the coating is thereby improved. At low stoving temperatures the proportion of organic contents is greater, that is to say the ductility of the coating becomes greater.

According to one aspect of the present invention an exhaust gas heat exchanger to be coated is force-flooded with the coating substance and then drained. The heat exchanger is then suspended so that all excess coating substance can run off without forming unwanted accumulations in the interior. Drops adhering to the outlet are suitably removed, for example by means of compressed air or with the aid of an electrostatic droplet extractor. The exhaust gas heat exchanger then runs through a drying oven.

The coating is here produced by a sol-gel process, for example by so-called ORMOCER layers. The term ORMOCER relates to a trademark of the Fraunhofer Gesellschaft for the advancement of applied research in Munich. To produce layers, organo(alkoxyl)silanes are purposely hydrolyzed through the use of suitable catalysts for the elimination of alcohols, such as methanol, ethanol, etc. Subsequent condensation reactions lead to the formation of organically modified inorganic-oxidic structures. For modification of the inorganic network, silicon can also be partially replaced by other elements, especially titanium, zirconium or aluminum. In addition the elements magnesium, zinc and calcium can be incorporated. The aqueous-alcoholic sols are applied to the heat exchanger to be coated in the flooding process, and heat-cured. In this way a cross-linked polymer layer is produced.

The purpose of the coating, in addition to the anticorrosive and/or water-repellent characteristic, is also at the same time to prevent dirt, particles, soot and oil films from adhering. When using ORMOCER layers this oleophobic effect can be achieved by a fraction of 0.1 to 10%, preferably 0.5 to 5% and in particular by 1 to 2% of fluorosilanes in the ORMOCER layer. The incorporation of a layer of simultaneously hydrophobic and oleophobic nature is achieved by the addition of an alkoxysilane with highly fluorinated alkyl chains. Particularly beneficial effects are achieved here by silanes of the F13(CF₃—(CF₂)₅—) and F16(CF₂H—(CF₂)₇—) type. 

1. A heat exchanger, in particular an exhaust gas heat exchanger, having at least one metal surface, in particular an aluminum or special steel surface, on which a medium, in particular exhaust gas, impinges and which is provided with a coating, wherein the coating comprises a coating material, which is based on nanotechnology.
 2. The heat exchanger as claimed in claim 1, wherein the coating comprises a main constituent, which is composed of an organic and an inorganic fraction.
 3. The heat exchanger as claimed in claim 1 wherein the coating contains silicon.
 4. The heat exchanger as claimed in claim 1, wherein the coating contains titanium, zirconium and/or aluminum.
 5. The heat exchanger as claimed in claim 1 wherein the coating contains magnesium, zinc and/or calcium.
 6. A method of manufacturing a heat exchanger, in particular an exhaust gas heat exchanger, as claimed in claim 1, wherein the coating is produced by a sol-gel process.
 7. The method as claimed in claim 6, wherein at least one sol is applied to the surface to be coated.
 8. The method as claimed in claim 7, wherein the sol is cured in order to form a cross-linked polymer layer.
 9. The method as claimed in claim 6, characterized by the following process steps: a) a heat exchanger to be coated is flooded with the coating material and drained; b) the drained heat exchanger is heated in a drying oven and/or has a flow of hot gas, in particular hot air, passed through it. 